The present disclosure is related to marking and printing methods and systems, and more specifically to automatically control the temperature of a Keyless inker for variable data lithographic printing based on pixel counting.
Offset lithography is a common method of printing today. (For the purpose hereof, the terms “printing” and “marking” are interchangeable.) In a typical lithographic process a printing plate, which may be a flat plate, the surface of a cylinder, belt, etcetera, is formed to have “image regions” formed of hydrophobic and oleophilic material, and “non-image regions” formed of a hydrophilic material. The image regions are regions corresponding to the areas on the final print (i.e., the target substrate) that are occupied by a printing or a marking material such as ink, whereas the non-image regions are the regions corresponding to the areas on the final print that are not occupied by the marking material.
The Variable Data Lithography (also referred to as Digital Lithography or Digital Offset) printing process begins with a fountain solution used to dampen a silicone imaging plate on an imaging drum. The fountain solution forms a film on the silicone plate that is on the order of about one (1) micron thick. The drum rotates to an exposure station where a high power laser imager is used to remove the fountain solution at the locations where the image pixels are to be formed. This forms a fountain solution based latent image. The drum then further rotates to a development station where lithographic ink is brought into contact with the fountain solution based latent image and ink develops onto the places where the laser has removed the fountain solution. The ink is hydrophobic. An ultra violet (UV) light may be applied so that photo-initiators in the ink may partially cure the ink to prepare it for high efficiency transfer to a print media such as paper or clothe. The drum then rotates to a transfer station where the ink is transferred to a printing media such as paper. The silicone plate is compliant, so an offset blanket is not used to aid transfer. UV light may be applied to the paper with ink to fully cure the ink on the paper. The ink is on the order of one (1) micron pile height on the paper.
The inking process is one of the main differences between traditional lithographic offset printing and variable data lithographic printing is that every image can be different, as in all digital printing. This is often referred to as Variable Data printing. Traditional lithographic offset is inherently a reprographic process in that all images for each revolution of the image drum are the same. Thus the mean ink throughput for each drum revolution is the same, and the critical ink-to-fountain-solution ratio is the same for each revolution. The process is tuned manually by an operator to find the correct ink supply rate to match the ink load and the image content, and to also match the ink and fountain solution mixture. The operator makes ink supply adjustments and visually inspects the printed output to perform the manual tuning process.
Skilled operators perform this function in traditional lithographic offset printing. Note that this is for a static document, which is a document that is the same for every revolution of the imaging drum. For variable data documents the challenge is even greater because the ink load presented by the image content is varying. If not enough ink is supplied, a starvation defect will occur similar to the xerographic defect known as “ghosting”, or “reload”. The final (or potentially intermediate) inker rollers cannot pick up enough ink from their upstream supply rollers to keep up with the demand presented by the image. Note that the ink trains and particularly the final inker rollers are somewhat robust to ink load variation in that the circumference of the ink rollers are typically many times smaller than the circumference of the image drum (which is the extent of the printed image). Thus the inker rollers are robust enough to handle local variations in the ink load, as long as the mean ink supply meets the mean ink load. Where the mean is taken, at least, over one imaging drum revolution. If too much ink is supplied to the inker rollers, then fine detail in the image will be washed out resulting in poor image quality. Again, the inker rollers are robust to this in that they can handle local variations in ink load for at least one revolution of the imaging drum. In traditional lithographic offset printing, once the operator has tuned the system for ink supply rate and fountain solution rate, the system reaches a quasi-equilibrium in that the mean ink load for one drum revolution is fixed and thus the required ink supply is fixed. The adjustments are made every 1 inch or so in the cross-process direction. Thus for a cross process location that does not have much image content such as text, a key setting associated with a low ink supply is chosen to match the ink load in that cross-process location. In a cross process location that has high image content such as a solid fill, a key setting associated with a high ink supply is chosen, again, to match the ink load in that cross-process location. Note that in all cross-process locations, whether they have high ink load or low ink load, the total ink load over an imaging drum revolution is the same (high, low, or in between) from revolution to revolution.
Typically, for a keyless inker in traditional lithographic printing the ink supply may be adjusted by manually adjusting the temperature of an ink bath. For Variable Data printing, the ink load will be different in both the cross-process and process directions. That is, the ink load will change in each process and cross-process location over time as the document is printed, since each page of the document can have different content. The ink supply system must automatically adjust to match ink supply with the dynamic ink load presented by the variable data image content. The ink baths are typically adjusted manually by a human operator when the printing unit is in a maintenance mode. While the past practices have been reasonably satisfactory, there is a need for an improved ink bath temperature adjustment mechanism so as to automatically adjust the ink flowing into in a variable data lithography system.